Line narrowed laser with bidirection beam expansion

ABSTRACT

A grating based line narrowing unit with bi-directional beam expansion for line narrowing lasers. In a preferred embodiment a beam from the chamber of the laser is expanded in the horizontal direction with a three-prism beam expander and is expanded in the vertical direction with a single prism. A narrow band of wavelengths in the expanded beam is reflected from a grating in a Littrow configuration back via the two beam expanders into the laser chamber for amplification.

[0001] This invention relates to lasers and in particular to linenarrowed excimer lasers. This invention is a continuation-in-part ofSer. No. 09/470,724, filed Dec. 22, 1999 and Ser. No. 09/716,041, filedNov. 17, 2000.

BACKGROUND OF THE INVENTION Narrow Band Gas Discharge Lasers

[0002] Gas discharge ultraviolet lasers used as light sources forintegrated circuit lithography typically are line narrowed. A preferredline narrowing prior art technique is to use a diffraction grating basedline narrowing unit along with an output coupler to form the laserresonant cavity. The gain medium within this cavity is produced byelectrical discharges into a circulating laser gas such as krypton,fluorine and neon (for a KrF laser); argon, fluorine and neon (for anArF laser); or fluorine and helium and/or neon (for an F₂ laser).

Prior Art Line-Narrowing Technique

[0003] A sketch of such a prior art system is shown in FIG. 1 which isextracted from Japan Patent No. 2,696,285. The system shown includesoutput coupler (or front mirror) 4, laser chamber 3, chamber windows 11,and a grating based line narrowing unit 7. The line narrowing unit 7 istypically provided on a lithography laser system as an easilyreplaceable unit and is sometimes called a “line narrowing package” or“LNP” for short. This unit includes two beam expanding prisms 27 and 29and a grating 16 disposed in a Litrow configuration so that diffractedbeam propagates right back towards the incoming beam. The output ofthese excimer lasers are typically rectangular with the long dimensionof for example 20 mm in the vertical direction and a short dimension offor example 3 mm in the horizontal direction. Therefore, in prior artdesigns, the beam is typically expanded in the horizontal direction sothat the FIG. 1 drawing would represent a top view.

The Grating Formula

[0004] Another prior art excimer laser system utilizing a diffractiongrating for spectrum line selection is shown in FIG. 2. The cavity ofthe laser is created by an output coupler 4 and a grating 16, whichworks as a reflector and a spectral selective element. Output coupler 4reflects a portion of the light back to the laser and transmits theother portion 6 which is the output of the laser. Prisms 8, 10 and 12form a beam expander, which expands the beam in the horizontal directionbefore it illuminates the grating. A mirror 14 is used to steer the beamas it propagates towards the grating, thus controlling the horizontalangle of incidence. The laser central wavelength is normally changed(tuned) by turning very slightly that mirror 14. A gain generation iscreated in chamber 3.

[0005] Diffraction grating 16 provides the wavelength selection byreflecting light with different wavelengths at different angles. Becauseof that only those light rays which are reflected back into the laserwill be amplified by the laser gain media, while all other light withdifferent wavelengths will be lost.

[0006] The diffraction grating in this prior art laser works in aLittrow configuration, when it reflects light back into the laser. Forthis configuration, the incident angle αand the wavelength λ are relatedthrough the formula:

2dnsinα=mλ  (1)

[0007] where α is the incidence angle on the grating, m is thediffraction order, n is refractive index of gas, and d is the period ofthe grating.

[0008] Because microlithography exposure lenses are very sensitive tochromatic abberations of the light source, it is required that the laserproduce light with very narrow spectrum line width. For example, stateof the art excimer lasers are now producing spectral linewidths on theorder of 0.5 pm as measured at full width at half maximum values andwith 95% of the light energy concentrated in the range of about 1.5 pm.New generations of microlithography exposure tools will require eventighter spectral requirements. In addition, it is very important thatthe laser central wavelength be maintained to very high accuracy aswell. In practice, it is required that the central wavelength ismaintained to better than 0.05-0.1 pm stability.

[0009] A need exists for greater narrowing of the laser beam.

SUMMARY OF THE INVENTION

[0010] The present invention provides for a grating based line narrowingunit with bi-directional beam expansion for line narrowing lasers. In apreferred embodiment a beam from the chamber of the laser is expanded inthe horizontal direction with a three-prism beam expander and isexpanded in the vertical direction with a single prism. A narrow band ofwavelengths in the expanded beam is reflected from a grating in aLittrow configuration back via the two beam expanders into the laserchamber for amplification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows a first prior art line narrowed laser system.

[0012]FIG. 2 shows a second prior art line narrowed laser system.

[0013]FIG. 3 shows the effect on wavelengths of vertical beam deviation.

[0014]FIGS. 4A, 4B and 4C show elements of a preferred embodiment of thepresent invention.

[0015]FIG. 5 shows beam expansion coefficient possible with one prism.

[0016]FIG. 6 shows a helium purge arrangement.

[0017]FIGS. 7, 8 and 8A-D show LNP's equipped for fast feedback control.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0018] Preferred embodiments of the present invention can be describedby reference to the drawings.

[0019] In reality, formula (1) presented in the Background Section onlyworks when all the beams incident on the grating have the same directionin the vertical axes, and this direction is normal to diffractiongrating grooves. Diffraction grating grooves are placed vertically soformula (1) works for beams which lay in the horizontal plane.

[0020] Real excimer laser beams, however, have some divergence in bothhorizontal and vertical directions. In this case, formula (1) ismodified and becomes

2dnsinα·cosβ=mλ  (2)

[0021] In this formula, β is the beam angle in the vertical direction,the rest of the variables are the same as in (1). In the case of β=O;i.e., when the beam has no divergence in the vertical direction, cosβ=1and formula (2) becomes (1).

[0022] It is important to note, that grating does not have anydispersion properties in the vertical direction, that is, its reflectionangle in the vertical direction does not depend on the light wavelength,but is rather equal to the incident angle. That means, in the verticaldirection the reflecting facets of the grating face are behaving likeordinary mirrors.

[0023] Beam divergence in the vertical direction has significant effecton line narrowing. According to formula (2), different vertical angles βwould correspond to different Littrow wavelengths λ. FIG. 3 showsdependence of Littrow wavelength λ on the beam vertical deviation, β.Typical prior art excimer laser might have a beam divergence of up to±1.0 mrad (i.e., a total beam divergence of about 2 mrad). FIG. 3 showsthat a portion of a beam propagating with a 1 mrad vertical tilt (ineither up or down direction) will have the Littrow wavelength shifted by0.1 pm to the short wavelength direction for that portion of the beam.This wavelength shift leads to broadening of the whole beam spectrum.Prior art excimer lasers, having Δλ_(FWHM) bandwidth of about 0.6 pmdoes not substantially suffer from this effect. However, as thebandwidth is reduced, this 0.1 pm shift becomes more important. Newexcimer laser specifications for microlithography will require bandwidthof about 0.4 pm or less. In this case, it becomes important to reducethis broadening effect.

First Preferred Embodiment

[0024] A preferred line narrowing module of the present invention isshown in FIGS. 4A, B and C. It has three beam expanding prisms thatexpand the beam in the horizontal direction and one additional prism,which expands the beam in the vertical direction.

[0025]FIG. 4A is a top view. FIG. 4B is a side view from the sideindicated in FIG. 4A. (In FIG. 4B the prisms are depicted as rectanglesrepresenting the portion of the prisms through which the center of thebeam passes.) FIG. 4C is a prospective view. Note that the grating 16and mirror 14 are at a higher elevation than prisms 8, 10, and 12. Notethat the expanded beam heads off in a direction out of the plane of thehorizontal beam expansion. The beam then is redirected back into asecond horizontal plane parallel to the plane of the horizontalexpansion by mirror 14 onto the face of the grating 16 which ispositioned in the Littrow configuration in the second horizontal plane.(Grating 16 is shown as a line in FIG. 4B representing the intersectionof the horizontal center of the beam with the grating surface.)

[0026] In the preferred embodiment, each of the three horizontallyexpanding prisms expands the beam by about 2.92 times. Therefore, totalbeam expansion in the horizontal direction is 2.923⁼25 times. The beamexpansion in the vertical direction is 1.5 times. (The degree ofexpansion is exaggerated in FIGS. 4B and C.) This vertical beamexpansion does not directly affect the beam divergence in the lasercavity or the vertical beam divergence of the output laser beam, but itdoes reduce the vertical divergence of the beam as it illuminates thegrating surface. After the beam is reflected from the grating, prism 60contracts the beam in its vertical direction as it passes back throughthe prism thus increasing its divergence back to normal. This reduceddivergence of the beam as it illuminates the grating results in areduction in the wavelength shift effect thus producing betterline-narrowing. A vertical tilt of 1 mrad of the beam before it goesthrough this prism is reduced to$\frac{1{mrad}}{1.5} = {0.67\quad {{mrad}.}}$

[0027] According to FIG. 3, this will correspond to wavelength shiftreduction from 0.1 pm to a mere 0.044 pm making this effectinsignificant for line narrowing of the next generation of lasers.

[0028] Persons skilled in the art will recognize that in addition to theabove-described specific embodiments of the present invention, there aremany other embodiments. For example, prism 60 can be placed before prism8, or between any two of prisms 8, 10, and 12. Prism combinations otherthan 3 prisms for horizontal beam expansion and 1 prism for verticalbeam expansion can be used as well. Techniques for substantially realtime control of several wavelength parameters are described in a UnitedStates patent application filed Sep. 3, 1999, Ser. No. 09/390,579 and ina United States patent application filed Oct. 31, 2000, Ser. No.09/703,317 which are incorporated by reference herein. These techniquesinclude fast feedback control of the position of the beam expandingprisms, grating curvature and tuning mirror position. Control of theposition of the laser chamber is also provided. FIG. 6 shows an LNP withhelium purge. FIG. 7 is a combination block diagram schematic drawing ofthe entire laser system and FIGS. 8A and 8B are drawings of the LNP withthe added feedback control features. In the FIG. 8 embodiment, thecurvature of the grating is controlled by grating curvature steppermotor 30 to compensate for the distortion caused by the hot gas layer onthe face of the grating. In the FIGS. 8A-D embodiment, the curvature ofgrating 82 is controlled with seven piezoelectric devices 86 actingthrough seven invar rods 84 against backing block 88 and compressionspring 90. This embodiment provides very fast adjustment of thecurvature of the grating face. FIG. 5 shows possible beam expansioncoefficients that can be achieved with a single prism by adjusting theincident angle.

[0029] The scope of the present invention should be determined by theappended claims and their legal equivalents.

We claim:
 1. A bidirection beam expansion line narrowing unit for alaser defining a laser chamber comprising: A) first direction beamexpander positioned to expand a beam from said laser in a firstdirection; B) a second direction beam expander positioned to expand saidbeam in a second direction; and C) a grating positioned to reflect aselected narrow band of wavelengths back, via said second direction beamexpander and said second direction beam expander, to said laser chamberfor amplication.
 2. A line narrowing unit as in claim 1 wherein saidfirst direction is horizontal and said second direction is vertical. 3.A line narrowing unit as in claim 1 wherein said first direction beamexpander is comprised of at least one prism and said second directionbeam expander is comprised of at least one prism.
 4. A line narrowingunit as in claim 1 wherein said first direction beam expander iscomprised of three prisms and said second direction beam expander iscomprised of a single prism.
 5. A line narrowing unit as in claim 1 andfurther comprising a tuning mirror.
 6. A narrow band excimer lasercomprising: A) a laser chamber comprising 1) two electrodes; 2) anexcimer laser gas; 3) a blower means for circulating the gas; 4) a pulsepower means for creating discharges between said electrodes to produceexcimer laser pulses; B) a resonant cavity comprising an output couplerand a line narrowing unit said line narrowing unit comprising; 1) firstdirection beam expander positioned to expand a beam from said laser in afirst direction; 2) a second direction beam expander positioned toexpand said beam in a second direction; and 3) a grating positioned toreflect a selected narrow band of wavelengths back, via said seconddirection beam expander and said second direction beam expander, to saidlaser chamber for amplication.
 7. A laser as in claim 1 wherein saidfirst direction is horizontal and said second direction is vertical. 8.A laser as in claim 1 wherein said first direction beam expander iscomprised of at least one prism and said second direction beam expanderis comprised of at least one prism.
 9. A laser as in claim 1 whereinsaid first direction beam expander is comprised of three prisms and saidsecond direction beam expander is comprised of a single prism.
 10. Alaser as in claim 1 and further comprising a tuning mirror.